1. Field of the Disclosed Embodiments
This disclosure relates to systems and methods for reading and writing ferroelectric memories and for implementing the reading and writing processes using organic or other thin-film transistors, where the reading is effected in a non-destructive manner to the stored data and/or the writing function is separated from the reading function in the writing/reading circuitry.
2. Related Art
Ferroelectric memories are non-volatile electric memory components that store information as remnant polarization in a ferroelectric material. A wide variety of usable ferroelectric materials exist. Often, the ferroelectric materials in ferroelectric memories are provided in the form of ferroelectric polymers including, for example, poly(vinylidenefluoride-co-trifluoroethylene) or P(VDF-TrFE), which tend to be very attractive in many ferroelectric memory applications based on the ease with which they can be physically manipulated and the ease with which the ferroelectric properties can be modified. Devices employing ferroelectric memories tend to have comparatively lower power usage and faster write performance than those using other memory technologies. They tend to support a greater maximum number of write-erase cycles. Also, they can be printed circuits. These advantages are balanced against certain disadvantages including lower storage densities, storage capacity limitations, and higher costs.
Typically, the ferroelectric capacitor constitutes an electronic device in which the ferroelectric material is sandwiched between two electrodes to form the capacitor with the ferroelectric material as the dielectric. In a simple, straightforward and conventionally-employed configuration, the ferroelectric capacitor will be in a parallel plate configuration, but other varied structures are possible and are often implemented.
Ferroelectric materials are characterized by having remnant polarization after an electric field has been applied and removed. A ferroelectric material has a nonlinear relationship between the applied electric field and the apparent stored charge. Specifically, the ferroelectric characteristic has the form of a hysteresis loop, which is very similar in shape to the hysteresis loop of ferromagnetic materials. Hysteresis loops associated with ferroelectric materials show that typically when a positive or negative electric field is applied across ferroelectric materials such as in a ferroelectric capacitor, a particular polarization response results. See FIG. 1 in the [056-0525] Application. If the applied electric field is of a sufficient magnitude, the capacitor will retain its polarization even after the field is removed. A ferroelectric capacitor is bistable, with two different polarization states being possible when no electric field is applied. The polarization state exhibited by the ferroelectric material, particularly as it is used in a ferroelectric memory, can be used to represent a single binary bit value of the data stored in the ferroelectric memory, i.e., a “1” or a “0”. A “set” value at either of these two stable points on the hysteresis loop for the particular ferroelectric material will remain stable when no electric field is presented after the data has been “written” to the ferroelectric material. For a more detailed discussion, see Naber et al., “Organic nonvolatile memory devices based on ferroelectricity,” Adv. Mater. 22, 2010, pp. 933-45 (hereinafter “Naber”), which is incorporated herein by reference describing the state of the art in ferroelectric memories in organic nonvolatile memory devices.
A bit of data is written to the ferroelectric memory by applying a bias across the ferroelectric material. A positive bias may write one state (“1”) value and a negative bias may write another state (“0”) value, or vice versa depending on a polarization of a ferroelectric memory. When an external electric field is applied across a ferroelectric material, dipoles in the material will tend to align themselves with the field direction, produced by small shifts in the positions of atoms and shifts in the distributions of electronic charge in the crystalline structure. After the charge is removed, the dipoles retain their polarization state. The binary values of “0” and “1” are thus stored as one of two possible electric polarizations in each ferroelectrically-based data storage cell.
In a typical configuration, data is stored according to a binary polarization state of the ferroelectric capacitor, as described above. Writing to the cells is typically accomplished by (1) applying a positive bias across the ferroelectric capacitor to write a “1”, or (2) applying a negative bias across the ferroelectric capacitor to write a “0”. Reading from the cells is typically accomplished by applying a negative bias across the ferroelectric capacitor and measuring the amount of charge released by the capacitor. This charge may be measured using one of a sense amplifier or a charge integrator, either of which may be used to convert the charge into a large voltage. The amount of charge measured depends on the polarization state held by the ferroelectric capacitor, with a larger charge magnitude corresponding to a “1” state and a smaller charge magnitude corresponding to a “0” state. It is important to note that the above description refers to the cell having “held” a charge, because the reading process is destructive. After the reading process, the cell typically always holds a “0” value. Once the cell has been read, if the cell held a “1,” the cell must be re-charged to that value again. Also, those of skill in the art recognize that, as used in the above discussion, the designations of “positive,” “negative,” “1,” and “0,” and their relationships to one another are arbitrarily assigned and that other combinations are appropriate.